1. Field of the Invention
The present invention relates to a blowing device for compressed air or the like having at least one feed channel which is connectable to a source of compressed air and an outlet which is shaped to impart to the compressed air a jet in the form of a ring, or part of a ring, under adiabatic expansion, and at least one communication channel adapted to connect the inside of the jet with the atmosphere.
2. Description of the Prior Art
The most common way to use compressed air for blowing purposes is by supplying the compressed air to a nozzle with one or several substantially circular outlet channels. The velocity of the discharge of the air is dependent upon the pressure upstream of the outlet channels and the pressure situation downstream of the same. If this pressure relation corresponds with the so-called critical pressure relation, the velocity of the discharge will be equal to the sound velocity. In most industries utilizing compressed air, the pressure normally present in the air supply network will be such, that the velocity of discharge, for example for cleaning purposes, using nozzles of the kind mentioned will be essentially equal to the sound velocity. Thus in most cases, the pressure relation will be equal to the critical pressure relation, i.e. 0.528.
When air is flowing out from an outlet in this manner under substantially adiabatic expansion there will occur a conically shaped core jet and outside of this a mixing zone where the air jet, due to transmission of movement to the ambient air in the form of expansion, will diverge and bring ambient air along with it in its movement. Thus, the air jet will increase in mass but will loose velocity. The loss of velocity entails that the dynamic pressure of the air jet will be partly transformed into static pressure. This pressure, added to the atmospheric pressure, comprises the counter pressure to which the pressure ratio is related.
The supply pressure at which critical flow occurs will thus be determined by the degree of co-ejection. From the point of view of co-ejection, among other things, it is an advantage to divide a given mass flow into several smaller part flows, so called multi-channel nozzles. This will provide, related to the mass flow amount, a considerably larger contact surface between outflowing air and ambient air, since the contact surface "KA" between outgoing flow and ambient air is directly proportional to the total circumference, O.sub.out, i.e. KA.dbd.O.sub.out .times.K. K is a constant which is determined, among other things, by the angle at which the air jet diverges, i.e. by the conditions of turbulance, and by the distance between the nozzle outlet and the work piece to which the air jet is directed.
For instance, in the case of 10 outlet channels with a diameter of 1 mm, O.sub.out =31.4 mm, whereas, for the same outlet area A.sub.out using 1 outlet channel, O.sub.out is less than 10 mm. Thus, the contact number KT, which may be expressed as O.sub.out /A.sub.out, will be 4 mm/mm.sup.2 and about 1.24 mm/mm.sup.2, respectively. One drawback of multi-channel nozzles is the manufacturing of the long and narrow channel. An increased O.sub.out, while maintaining the same A.sub.out, to for instance 2 times 31.4 mm, i.e. to an increased KT of 8 mm/mm.sup.2, will necessitate 40 channels with a diameter of 0.5 mm. Such a nozzle outlet, which gives a lower noise level, is difficult to implement in view of the manufacturing.
At the normal supply pressures of 6-8 bar there is obtained at larger nozzle outlets, preferably larger than 40 mm.sup.2, a counter pressure which is lower than 0.528 times the supply pressure.
Within an outgoing air jet there will occur downstreams of the outlet local differences in velocity, pressure and density. The locally and periodically varying pressure differences will be reduced at a reduced outlet cross section. From the point of view of noise it is for instance known, that it is an advantage to divide a larger flow into several smaller and well distributed flows.
However, if the outlet channels in a multi-channel nozzle are placed too close to one another--for instance when there is a demand for larger mass flows--the atmospheric air will be prevented from communicating with the central portions within the generated jet bundle in a satisfying manner. Such communication is a prerequisite for, among other things, a low noise level in these nozzle embodiments.
Another common type of nozzle is the so called ejector nozzles which are commonly used for cooling, drying, and above all to blow away smoke or exhaust gases. The ejector nozzles, for instance in accordance with the Swedish patent specification SE.A. No. 8000567-1, operate by co-ejection via the central portions of the nozzle and remove smoke or exhaust gases from for instance a welding work place. The outgoing flow has a low power concentration and is strongly turbulent. This is caused by the fact that the trough-flow area of the common central outlet is much larger and by the fact that the friction losses within the outlet channel are extremely high. The frequencies spectrum of the resultant noize differs markedly from conventional blow nozzles.
For instance, it is known that pressurized outflowing gas gives a dominant noise generation at the so called Strouhal frequency, fs, which is determined by the relation SN.times.u/d, where
SN=The Strouhal number which at a Reynold's number of &gt;500 is equal to 0.2 (dim. less) PA1 u=outflow velocity, m/s PA1 d=cross-sectional dimension (s), m
For instance, in a circular outlet with an outlet diameter of 10 mm, there will be obtained, at normal critical outflow of air, a dominant noise generation within the frequency range of 6-7 kHz. At lower outlet velocities, for instance in ejector nozzles, a dominant noise generation will occur at substantially lower frequencies. With the outlet dimensions normally present in ejector nozzles, 10-75 mm, the dominant noise generation is at frequencies which are especially damaging to the human ear, or from about 4 kHz at the smaller outlet dimensions to about 1 kHz at the larger outlet dimension.
If, in an annularly shaped slit orifice, the ratio between the velocity of flow and the slit width is sufficiently large, dominant noise generation occurring at the outlet may be displaced to higher frequencies which are outside the range of frequency audible to humans. However, the vacuum generated in the central portions of the air jet will give rise to such a turbulent flow, that minimizing of the slit will not result in any substantial noise reduction in the surrounding area of these types of nozzles. Filling up a vacuum space with a solid body, for instance in accordance with the U.S. Pat. No. 3,984,054 does not result in any substantial improvements with regard to the noise.
The commerically available blow nozzles differ widely as concerns the blowing power. Since furthermore the need of blowing power varies considerably from one work place to another, and also within one and the same work place, and since neither the conventional nozzles and complete blowing tools are possible to regulate, nor are provided with information about the blowing power, the purchase and installation of such blowing devices involves many problems. The consequence is that the blowing devices will mostly have a too large capacity. Thus in most cases the air consumption, the noise and the risk of injury will be unnecessarily high.
A blowing tool of conventional type has a valve or regulation arrangement the blow-through area of which is substantially directly proportional to the displacement of the valve or regulator element. Since the blow-through amount at the outlet is a function of the area ratio between the blow-through areas at the valve and at the outlet, and since this function is very unlinear, the possibility of a control regulation of the amount of flow will be limited.
Displacement of the valve body from the closed position only a few tenths of a millimeter results in multiple changes of the amount of flow through the blowing device. On the other hand, a corresponding valve displacement at a position of larger opening will only result in percent changes of the amount of flow.
In the often reccuring work of blowing away dirt form machines, manufactured parts etc., additional noise is caused when the flowing gas hits the object to be cleaned. When cleaning so called bottom holes, a noise situation occurs which is completely dominated by the generation of sound at the hole. This type of work, which is mostly performed manually, gives rize to sound levels which at a distance of one meter generally exeeds 100 dB(A). The work also causes chips and cutting fluid to be squirted around. Such squirting of chips and cutting fluid causes a lot of eye injuries to the user as well as to persons in the vicinity.
The noise as well as the risk of squirting around chips may be reduced a certain amount by the aid of previously known technics, for example according to the German Pat. No. 2,908,004. However, this type of design has the considerable drawback that the gas fluid exiting from the centrally located exhaust tube will often obtain a hit zone which is outside of the hole to be blown clean. The operator therefore has to move the nozzle, by means of sweeping movements, to a position where the outflow of gas from the exhaust tube is located directly above the hole. The smaller the hole is, the longer time is needed for finding the correct position. Furthermore, such sweeping movements also entails that the operator will momentarily raise the plane of the nozzle from the object to be cleaned in order to reduce the friction between the end of the nozzle which mostly is made of rubber, and the object. The flow of gas through the slot thereby formed results in very high noise levels and, in certain cases, severe squirting of cutting fluid.
The drawbacks mentioned may be reduced if the exhaust tube is placed outside of the nozzle plane. However, this placement causes the exhaust tube as well as the object to be cleaned to be subjected to mechanical abrasion. The abrasion of the exhaust tube is especially high in connection with threaded hole configurations. In most manufacturing processes no mechanical abrasion, i.e. scratches, on the manufactured part are acceptable. Another drawback with an exhaust tube projecting from the nozzle is that this design is not usable at smaller hole diameters. In a threaded bottom hole, as an example, the diameter of the hole generally has to be larger than 6 mm.
A very important inconvenience in the cleaning of bottom holes according to the technic mentioned is the absence of an extensive regulation of the amount of stream. Different hole depths, hole configurations, cutting fluids etc. give rise to greatly different requirements as concerns the blowing power.